Human polo-like kinase 1 (human PLK1) is a Serine/Threonine kinase, which belongs to a PLK family, named after a structurally similar polo kinase from Drosophila melanogaster. Drosophila polo mutants are defective in mitosis and have abnormal mitotic spindle poles. In mammals, four structurally homologous PLK proteins (PLK1, PLK2, PLK3, and PLK4) have been identified to date, all of them sharing a common architecture and having an N-terminal kinase catalytic domain and a C-terminal region containing either one (PLK4) or two (PLK1, PLK2, PLK3) polo-box domains. PLK1 is the most studied and best characterized member of the family. Specifically, PLK1 recapitulates most of the functions of polo kinase and is known to be a key regulator of mitosis in human cells.
PLK1 expression level and activity are strongly cell-cycle regulated, and peak around the time the cell enters into mitotic phase. Importantly, PLK1 transcripts are found only in proliferating tissues, while PLK2 (Serum-inducible kinase, SNK) and PLK3 (Proliferation-related, fibroblast growth factor inducible kinase, FNK) transcripts have broad tissue distribution, including post-mitotic neurons. PLK4/SAK is believed to function mostly in centriole biogenesis, and, like PLK1, is essential for cell viability.
PLK1 is essential for normal mitotic progression, and functions at multiple steps of mitosis. Specifically, PLK1 is necessary for entry into mitosis, for centrosomal maturation, mitotic spindle assembly and maintenance, exit from mitosis and for cytokinesis. At each of these steps PLK1 phosphorylates a distinct set of substrate proteins. Before entry into mitosis PLK1 phosphorylates and activates phosphatase cdc25c, which then translocates to the nucleus and, in turn, removes inhibitory phosphates from cyclin-dependent kinase CDK1. PLK1 also phosphorylates cyclin B1, the partner of CDK1. Together, activated CDK1/cyclin B complex initiates entry of the cell into mitosis.
PLK1 overexpression is believed to be strongly associated with neoplastic cells. Specifically, it has been shown that PLK1 RNA is expressed at high levels in lung and breast tumors, with little to no expression in adjacent normal tissue. Further, PLK1 overexpression was found to correlate with histological grade, and poor prognosis in several types of cancer, such as ovarian and endometrial cancers, esophageal carcinomas and non-small cell lung carcinomas.
Downregulation of PLK1 in tumor cells induces mitotic arrest and subsequent cell death. Therefore what is needed are compounds and compositions that inhibit PLK1 for use in treatment of disease states where the PLK1 pathway is involved.